General relativity survives gruelling pulsar test

Astronomers have used a pair of pulsars orbiting each other, found with CSIRO

They also hope to be able to use the two pulsars to determine the exact nature of the matter that pulsars and other neutron stars are made of.

An international research team led by Professor Michael Kramer of the University of Manchester's Jodrell Bank Observatory, UK, has been observing the double-pulsar system since 2003 with three of the world’s largest radio telescopes: CSIRO’s Parkes radio telescope in NSW, Australia; the Lovell Telescope near Manchester, UK; and the Robert C. Byrd Green Bank Telescope in West Virginia, USA.

The double-pulsar system, whose pulsars are called PSR J0737-3039A and B, is the only known system of radio pulsars orbiting each other. It lies 2000 light-years away in the direction of the constellation Puppis.

The system consists of two massive, highly compact neutron stars, each weighing more than our own Sun but only about 20 km across, orbiting each other every 2.4 hours at speeds of a million kilometres per hour.

Separated by a distance of just a million kilometres, both neutron stars emit lighthouse-like beams of radio waves that are seen as radio ’pulses‘ every time the beams sweep past Earth.

By precisely measuring the variations in pulse arrival times, the researchers found the movement of the stars to exactly follow Einstein's predictions. 'This is the most stringent test ever made of GR in the presence of very strong gravitational fields, only black holes show stronger gravitational effects, but they are obviously much more difficult to observe,” Professor Kramer says.

“The double-pulsar system, whose pulsars are called PSR J0737-3039A and B, is the only known system of radio pulsars orbiting each other. It lies 2000 light-years away in the direction of the constellation Puppis.”Co-author Ingrid Stairs, an assistant professor at the University of British Columbia in Vancouver, Canada, says it is possible to measure the pulsars’ distances from their common centre of mass. 'The heavier pulsar is closer to the centre of mass, or pivot point, than the lighter one and this allows us to calculate the ratio of the two masses,” she says.

This mass ratio is independent of the theory of gravity, and so tightens the constraints on general relativity and any alternative gravitational theories.

Other relativistic effects predicted by Einstein can be observed: the fabric of space-time around pulsar B is curved, and the other pulsar’s “clock” runs slower when it is deeper in the gravitational field of its massive companion. Each of these effects provides an independent test of general relativity.

The distance between the pulsars is shrinking by 7 mm a day. Einstein's theory predicts that the double pulsar system should be emitting gravitational waves, ripples in space-time that spread out across the Universe at the speed of light.

'These waves have yet to be directly detected,” says team member Prof. Dick Manchester of CSIRO’s Australia Telescope National Facility ATNF). 'But, as a result, the double pulsar system should lose energy causing the two neutron stars to spiral in towards each other by precisely the amount that we have observed, thus our observations give an indirect proof of the existence of gravitational waves.'

The astronomers hope that over the next few years they can make even more precise measurements of the characteristics of the system, allowing them to measure the moment of inertia of a neutron star. (“Moment of inertia” is a measure of how much a body resists a force trying to rotate it.) “This measurement may be very difficult but if we could do it to just a precision of 30 per cent, we could distinguish between the many different ideas about the nature of the matter that makes up neutron stars,” says team member Dr George Hobbs of the ATNF.

Technical note
Six parameters are measured in the tests of general relativity. They relate to:

the relativistic precession of the orbit
variations in the Doppler effect and gravitational redshift as the pulsar moves around its elliptical orbit
the time variation in the orbital period
the Shapiro delay, which describes a delay to a pulse travelling through the curved space-time of a massive object, and
the mass ratio derived from the measured semi-major axes of the orbits.